Thanks for everyone's suggestions.
I decided to crack it open for the 2nd time and go for a mechanical checkout on the BNC connectors and "everything connected to them in the close vicinity.
Although I initially suspected the non-optimal soldering of the central BNC pin that proved to be a red herring.
Getting the old stereo microscope gave, literally, a whole new perspective. I managed to take some pictures via the microscope and let them speak for themselves..
Interestingly enough only K700 and K800 were affected, the other 2 relays soldering appeared OK. In any case I resoldered all relay pins and that seems to have fixed it. Using real solder, the Pb/Sn version I mean, not that newfangled RoHS rubbish ??
I'll keep the patient under observation of course but for now it seems to have been cured. A user cal has been completed successfully.
Wilko
